The present invention is directed to signal-acquisition systems. It is concerned specifically with a system for processing the output of a circular array of antenna elements so as to determine both the azimuth and the elevation angles of the source of signals that the antenna array receives.
U.S. patent application Ser. No. 551,664, filed on Nov. 14, 1983, by Apostolos, Boland, and Stromswold for an ACQUISITION SYSTEM EMPLOYING CIRCULAR ARRAY, discloses a powerful system for determining the directions of arrival and frequencies of many signals simultaneously. An improvement in that system is disclosed in U.S. patent application Ser. No. 536,477, filed on Sept. 28, 1983, by John T. Apostolos for a TWO-DIMENSIONAL ACQUISITION SYSTEM USING CIRCULAR ARRAY. In both of these systems, a spatial Fourier transformation is performed on the outputs of a circular antenna array. The resultant transform is processed with certain correction factors related to the antenna pattern of the array and then subjected again to a spatial Fourier transformation. A temporal Fourier transformation is also performed. The result of each system is an ensemble of signals at a group of output ports in which each output port represents a different azimuthal direction. Signals from a source in a given azimuthal direction result in a maximum output at the port associated with that azimuthal direction. Thus, the azimuthal direction of each source is readily identified in real time. The descriptions included in these patent applications are helpful in understanding the present invention, and they are accordingly incorporated by reference.
The assumption on which the design of the systems of those two applications is based is that the source has a negligible elevation angle. That is, there is only a very small angle between the direction of arrival of the signal and the plane of the circular antenna array. For a wide range of applications, this is an accurate assumption. For sources whose angle of elevation is significant, however, the direction indications produced by the systems of those two applications are inaccurate.
An object of the present invention is to eliminate the inaccuracies that can be caused in such systems by significant elevation angles.
It is a further object of the present invention to determine the values of the elevation angles of signal sources.